I'm sorry but people do not walk across London to change trains. No-one walks from Fenchurch Street to Victoria, from London Bridge to Euston, from Marylebone to Kings Cross to change trains.
There will be a very,very tiny number of people who do - I've walked Charing Cross/Waterloo to Euston, Paddington to Marylebone/Victoria/Charing Cross myself on occasions.
The vast, vast majority will uses tubes, buses, taxis or cycle.